The present invention relates to magnetic flow meters for measuring flow velocity of a conductive fluid. More particularly, the invention provides an improved inductive-type magnetic flow meter.
Magnetic flow meters typically include a conduit, such as a tube, through which a conductive fluid, whose flow velocity is to be measured, flows. A field generator, such as an electromagnetic coil, generates a magnetic field within a portion of the fluid flow and directed transverse to the direction of the flow. Such flow meters further include a pair of electrodes selectively spaced apart relative to the direction of the fluid and the direction of the applied magnetic field. The passage of the fluid through the magnetic field induces a voltage, i.e., a potential difference, between the paired electrodes. This voltage is proportional to the strength of the applied magnetic field, to the flow velocity of the fluid, and the separation of the electrodes. This typical magnetic flow meter further includes an evaluation circuit that measures the strength of the applied magnetic field and the induced voltage at the electrodes, to provide the flow velocity of the fluid.
Early magnetic flow meters employed alternating current (AC) for generating an AC magnetic field within the fluid. In these flow meters, the field generator, e.g., a coil, was typically directly connected to an AC power supply, operating, for example, at 220V/50 Hz. Such AC flow meters suffered from susceptibility to zero drift and from considerable reactive power consumption.
Subsequent introduction of pulsed DC magnetic flow meters solved some disadvantages of AC flow meters. In pulsed DC flow meters, pulses of DC current excite a coil, which in turn generates pulses of magnetic field within the fluid. Although pulsed DC flow meters have low power consumption and good zero stability, they are typically not adequate for high noise applications. In particular, the excitation current in these flow meters is typically small, resulting in small magnetic fields and hence low signal amplitudes, e.g., low magnitude of the induced voltage between the electrodes.
U.S. Pat. Nos. 5,641,914 and 5,808,208, the disclosures of which are incorporated herein by this reference, describe a pulsed DC magnetic flow meter with relatively high coil excitation current. The disclosed flow meter employs numerous analog components, which render the construction of the flow meter cumbersome and degrade its performance characteristics.
Accordingly, it is an object of the present invention to provide an improved inductive-type flow meter.
This invention provides an electrical inductive flow meter circuit for evaluating flow velocity of a conductive flowing fluid. The electrical circuit of the invention includes an evaluation channel for measuring a voltage induced in the flowing fluid as the fluid passes a region having a pulsed magnetic field. The magnetic field, which is directed transverse to the direction of fluid flow, induces the voltage across the fluid in a direction transverse to both the field direction and the direction of the fluid flow. The circuit has a reference channel for providing a reference voltage indicative of the amplitude of the pulsed magnetic field.
The electrical circuit of the invention includes a pair of analog-to-digital converters (ADC) that are active concurrently during at least selected intervals of a measurement cycle. A measurement cycle is selected to include at least the duration of the pulsed magnetic field. One ADC is connected to the evaluation channel to receive the value of the induced voltage. The other ADC is connected to the reference channel to receive the value of the reference voltage. A programmable controller, such as a micro-controller, activates the converters during selected intervals of a measurement cycle so that the converters receive, concurrent, and preferably simultaneous, values of the induced voltage and of the reference voltage, and digitize them.
The programmable controller employs a stable clock for timing the measurement cycle. In particular, the controller activates the converters concurrently during selected intervals of the measurement cycle, and receives the digital values of the induced voltage and the reference voltage that the converters provide. The controller employs these values to calculate the flow velocity of the fluid.
In one aspect, the electrical flow-meter circuit includes a coil for generating the magnetic field in the flow region of the fluid. A coil excitor energizes the coil with a succession of direct current (DC) pulses of selected duration and repetition, to generate the pulsed magnetic field.
One preferred embodiment of the invention employs a pair of electrodes spaced apart transverse to the flow direction and transverse to the magnetic field for producing electrical signals indicative of the voltage induced across the flowing fluid. The evaluation channel receives the electrical signals at the electrodes, and can include two integrating amplifiers, each connected to one electrode to amplify these signals. The evaluation channel can further include a differential amplifier which receives the output signals of the two integrating amplifiers, to provide an induced voltage proportional to a difference between the signals induced at the electrodes.
A third integrating amplifier receives the induced voltage and, in response, applies an amplified voltage to one analog-to-digital converter during an active interval, to produce a digitized voltage differential. One preferred embodiment of the circuit optionally includes a reference circuit that imposes a reference voltage, preferably selected to be substantially zero, at the input of the third integrating amplifier at the beginning of each measurement cycle, to reduce DC drift from a previous measurement cycle.
The reference circuit can include a voltage summer which receives the induced voltage at one input and is connected at its output to an input of the third integrating amplifier. The reference circuit further includes an integrating circuit connected between the output of the third integrating amplifier and another input of the summer. The integrating circuit is preferably configured to have a relatively long time constant, illustratively four seconds in one embodiment, to ensure that the momentary closure of the switch S1 results in substantially zeroing the DC offset voltage accumulated at the input of the amplifier 34 from a previous measurement cycle.
According to a further aspect of the invention, the reference channel includes a current sensor that is connected to a coil that generates the magnetic field. The sensor measures the current through the coil and provides the measured value of the current to an integrating amplifier. The integrating amplifier provides a reference voltage that is indicative of the measured current, and is hence indicative of the strength of the magnetic field. One of the analog-to-digital converters receives this reference voltage during an active interval, and provides a digitized value of the reference voltage to the programmable controller.
In one preferred embodiment of the circuit of the invention, a current regulator is connected to the coil exciter and stabilizes the DC current pulse that generates the pulsed magnetic field, thereby stabilizing the magnitude of the magnetic field. This stabilization advantageously improves the signal-to-noise ratio of the measured induced voltage differential, thereby resulting in a more accurate measurement of the flow velocity of the fluid.